The subject invention relates to optical metrology devices which require a controlled ambient atmosphere to improve measurement. More specifically, the invention relates to an optical metrology instrument that includes a gas-flow system for the purpose of purging the optical path to stabilize measurement and clear the path of optically absorbing species.
The semiconductor industry is presently developing photolithographic methods utilizing 157 nm wavelength laser light as the next step in the continuing reduction of device length scales. Metrology tools are presently needed to support this development, for instance by providing measurements of the optical properties of candidate materials over the spectral range from xcx9c140 to xcx9c200 nm. These wavelengths lie within a region known as the vacuum ultraviolet (VUV), in which the high absorption coefficients of oxygen and water vapor lower the attenuation length in standard air to fractions of a millimeter. (Historically, this light could only be observed under vacuum conditions, hence the designation.) Achieving the transmission and stability necessary for a VUV optical metrology tool, in which the optical paths are 0.5-2 m, therefore requires oxygen and water concentrations in the low parts-per-million (ppm) range averaged over the entire optical path. In the near future, as production facilities incorporating 157 nm lithography come online, larger numbers of these toolsxe2x80x94capable of handling production line throughputxe2x80x94will be required. A major engineering challenge in the development of 157 nm metrology is simultaneously providing high wafer throughput and low optical absorption.
In the prior art, Freeouf, in U.S. Pat. No. 6,222,199 B1, incorporated herein by reference, teaches the benefits of performing specular bi-directional ellipsometric measurements in a geometry where the entire light path is maintained in a controlled ambient to prevent absorption and local excitation. At present, multiple commercial manufacturers offer VUV spectroscopic ellipsometer (SE) products that maintain a controlled ambient via housing the entire metrology apparatus inside a sealed container filled (purged) with purified nitrogen gas.
A design that places the complete instrument in a sealed container has two notable disadvantages. First, the purged volume is significantly larger than the volume that encloses the VUV optical path alone and must house multiple componentsxe2x80x94e.g., optical elements, optical mounts, electrical components, electrical wiring, actuators, adhesives, etc.xe2x80x94which do not need to be in the purged atmosphere. This places stringent and overly restrictive requirements on component materials since component and material out-gassing can degrade the purge environment. Furthermore, a high volume purge-gas flow is required to cool the system electronics. Second, the system requires some sort of sealable entry port or load-lock to enable sample (wafer) insertion while preventing the introduction of oxygen and water vapor contaminants into the chamber. This necessarily hinders wafer handling and substantially reduces throughput and usability.
Accordingly it would be desirable to provide a VUV metrology tool architecture wherein the purged volume is minimizedxe2x80x94approaching the volume of the VUV optical pathxe2x80x94and the instrument does not require a load-lock to isolate either the sample (wafer) or the metrology tool from the laboratory environment.
The subject invention relates to a VUV optical metrology system that incorporates a gas-purge of the optical pathxe2x80x94the light path that connects the illuminator, the sample and the detector. The metrology system avoids the use of a continuous, solid, barrier to separate the optical path from the laboratory environment; therefore, no load-lock is required. A substantially oxygen and water-vapor free environment is created and maintained by hydrodynamic flow of purified inert purge-gas that is introduced at least one injection point along the optical path.
The flow acts to displace optically adsorbing contaminants from the optical path, remove optically absorbing species from the surfaces bounding the optical path and, inhibit back diffusion of the chemical contaminants displaced in the flow. The purge-gas flow simultaneously prevents back-diffusion of atmospheric constituents from the laboratory environment to the optical path. This is achieved using a geometrical arrangement where the system optics is maintained in a housing that has a substantially planar surface.
The measurement process is initiated by raising the wafer from a load position to a measurement position where the wafer surface is substantially parallel to the housing surface where light is incident on the wafer at a measurement location. In the measurement position the volume bounded by the wafer and the housing approximates a thin disk. The flow system is arranged such that purge-gas is injected into the bounded volume at flow sufficient to purge the bounded volume in the vicinity of the measurement location. The purge gas flows outward from the measurement location and is exhausted at the wafer boundary.
The subject purge system will also improve measurements at longer wavelengths. More specifically, the purge system will help to stabilize temperature and make measurements conditions more uniform. This will lead to more accurate and repeatable measurements.